In recent years, demands for larger capacity and higher speed in wireless communications have increased, resulting in quite a number of studies being carried out to improve the effective utilization rate of finite frequency resources. One method of improving the effective utilization rate of frequency resources that is attracting attention involves utilizing space domains. An adaptive array antenna (adaptive antenna) is a technique for using space domains. An adaptive array antenna adjusts an amplitude and a phase of a reception signal through the use of a weighting coefficient (hereinafter, the weighting coefficient shall be referred to as a “weight”) to be multiplied onto the reception signal so as to strongly receive a signal arriving from a desired direction and suppress a signal arriving from a direction of interference waves. Accordingly, a communication capacity of a system can be improved.
Other techniques that utilize space domains take advantage of a spatial orthogonality of a propagation path to transmit different data series using a physical channel having the same time, same frequency, and same sign (hereinafter referred to as a “same physical channel”). Such techniques include: (1) space division multiple access (hereinafter referred to as “SDMA”) that uses the same physical channel to transmit different data series to different terminal devices; and (2) spatial multiplexing (hereinafter referred to as “SDM”) that uses the same physical channel to transmit different data series to the same terminal device.
SDMA technique is described in, for example, “A study on a channel allocation scheme with an adaptive array in SDMA” (T. Ohgane et al., IEEE 47th VTC, Page(s): 725-729, vol. 2 (1997)). SDMA can be implemented if a spatial correlation coefficient between terminal devices is lower than a predetermined value. SDMA enables an improvement in throughput and an increase in simultaneous transmission capacity of a wireless communication system.
SDM technique is described in, for example, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas” (G. J. Foschini, Bell labs Tech. J, pp. 41-59, Autumn 1996). A transmitter and a receiver both comprise a plurality of antenna elements and are capable of realizing SDM transmission in a propagation environment where correlation of reception signals between antennas is low. The transmitter transmits, for each antenna element, a different data series using a same physical channel. The receiver separates and receives reception signals based on different data series received by a plurality of antennas. By using a plurality of spatial multiplexing channels in this manner, data transmission rate can be increased without having to use multilevel modulation. When performing an SDM transmission, in an environment where a large number of scatterers exist between a transmitter and a receiver under sufficient S/N (signal-to-noise ratio) conditions, communication capacity can be expanded in proportion to the number of antennas (the transmitter and the receiver have the same number of antennas).
Multiuser MIMO is a technique that merges the SDMA and SDM techniques described above. A multiuser MIMO technique is described in, for example, “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser MIMO channels” (Q. Spencer et al., IEEE Trans. SP, Vol. 52, No. 2, pp. 461-471, 2004). The multiuser MIMO technique enables space division multiple access due to spatial multiplexing and directionality under a condition where a channel matrix of a concurrently-connected receiver is known by a transmitter. Due to the multiuser MIMO technique, even when the number of antennas of the receiver is limited, an improvement in throughput and an increase in simultaneous transmission capacity of a wireless communication system can be realized by using a multiuser antenna or by appropriately selecting combinations of antennas with respect to a plurality of receivers existing in a communication area. As a method of further improving an effective frequency utilization efficiency of the multiuser MIMO technique, for example, a method that is an application of dirty paper coding (hereinafter referred to as “DPC”) described in “Writing on dirty paper” (M. Costa, IEEE Trans. Inform. Theory, vol. 29, pp. 439-441, May 1983) has been proposed in “An Introduction to the Multi-User MIMO Downlink”, (Quentin H. Spencer, Christian B. Peel, A. Lee Swindlehurst, Martin Haardt, IEEE Communications Magazine, Vol. 42, Issue 10, pp. 60-67, October, 2004). The document describes that throughput can be significantly improved under ideal conditions by applying DPC to multiuser MIMO technique.